TRANSPLANTATION OF CELL NUCLEI
33
from the nucleus to the cytoplasm and back again into the nucleus
(Goldstein, 1958). A labelled nucleus from an amoeba incubated with
lysine-C
14
is transplanted to an unlabelled non-enucleated recipient
amoeba. After about 5 h one-third of the total radioactivity has moved
to the host nucleus, but hardly any is seen in the cytoplasm. If the host
nucleus which has accumulated the label from the injected nucleus is
then transplanted again to another unlabelled amoeba, the label then
distributes itself equally between the two nuclei (Goldstein, 1963a). Since
only one-third of the initially labelled substance migrates to the host
nucleus, there must be a migrating as well as non-migrating substance.
The migrating substance is a macromolecular protein, since its relative
location in the nucleus after serial host-nucleus transfers could not be
explained if it consisted of free amino acids. As Goldstein points out, the
behaviour of these substances suggests that they may be involved in
the control of gene activity by the cytoplasm. This would then be a kind
of intracellular communication for which direct evidence has not previously been provided, but which a wide range of embryological experiments has shown must take place throughout development.
IV. Conclusions from Nuclear Transfer Experiments Compared with
those from other Experiments
A. Variation in the Genetic Material of Differentiating Cells
We have concluded from the nuclear transfer experiments discussed
in Section III, A, 2, that mutational gene changes do not play an
important part in cell differentiation. This conclusion can be arrived at
directly in plants since gametes can be obtained from somatic cells
(examples in Sinnott, 1960). In animals new individuals can only be
obtained from somatic cells by nuclear transplantation. We will now
consider whether this conclusion is consistent with the fact that chromosome changes are known to occur regularly in the normal development
of certain organisms.
Since differentiated cells rarely divide except during regeneration
when the dividing cells may not be typical of the rest, useful chromosome
studies cannot usually be made on differentiated somatic cells of animals.
Indirect evidence, such as nuclear size or light absorption by stained
nuclei, indicates that most cell types are diploid in plants and animals.
However, liver cells of mammals and probably of other vertebrates are
known to range from diploid to octaploid (Glass, 1956). Polyploidy is
also regularly associated with the differentiation of insect nerve cells,
plant hair cells, etc. (review by Stern, 1958). A special type of polyploidy occurs in those families of Diptera which have polytene chromosomes (Section IV, C). Another peculiarity of insect development occurs
33
from the nucleus to the cytoplasm and back again into the nucleus
(Goldstein, 1958). A labelled nucleus from an amoeba incubated with
lysine-C
14
is transplanted to an unlabelled non-enucleated recipient
amoeba. After about 5 h one-third of the total radioactivity has moved
to the host nucleus, but hardly any is seen in the cytoplasm. If the host
nucleus which has accumulated the label from the injected nucleus is
then transplanted again to another unlabelled amoeba, the label then
distributes itself equally between the two nuclei (Goldstein, 1963a). Since
only one-third of the initially labelled substance migrates to the host
nucleus, there must be a migrating as well as non-migrating substance.
The migrating substance is a macromolecular protein, since its relative
location in the nucleus after serial host-nucleus transfers could not be
explained if it consisted of free amino acids. As Goldstein points out, the
behaviour of these substances suggests that they may be involved in
the control of gene activity by the cytoplasm. This would then be a kind
of intracellular communication for which direct evidence has not previously been provided, but which a wide range of embryological experiments has shown must take place throughout development.
IV. Conclusions from Nuclear Transfer Experiments Compared with
those from other Experiments
A. Variation in the Genetic Material of Differentiating Cells
We have concluded from the nuclear transfer experiments discussed
in Section III, A, 2, that mutational gene changes do not play an
important part in cell differentiation. This conclusion can be arrived at
directly in plants since gametes can be obtained from somatic cells
(examples in Sinnott, 1960). In animals new individuals can only be
obtained from somatic cells by nuclear transplantation. We will now
consider whether this conclusion is consistent with the fact that chromosome changes are known to occur regularly in the normal development
of certain organisms.
Since differentiated cells rarely divide except during regeneration
when the dividing cells may not be typical of the rest, useful chromosome
studies cannot usually be made on differentiated somatic cells of animals.
Indirect evidence, such as nuclear size or light absorption by stained
nuclei, indicates that most cell types are diploid in plants and animals.
However, liver cells of mammals and probably of other vertebrates are
known to range from diploid to octaploid (Glass, 1956). Polyploidy is
also regularly associated with the differentiation of insect nerve cells,
plant hair cells, etc. (review by Stern, 1958). A special type of polyploidy occurs in those families of Diptera which have polytene chromosomes (Section IV, C). Another peculiarity of insect development occurs